1. Field of the Invention
The present invention relates to a printed circuit board, and in particular to a printed circuit board used in electronic devices such as, for example, personal computers, copiers, printers and fax machines.
2. Description of the Related Art
In order to transmit digital signals on printed circuit boards used in electronic devices, it is necessary to be able to sufficiently transmit digital signals with harmonic waves of ordinarily up to five times or greater with respect to the frequency of fundamental waves of a binary pulse. Namely, in order to transmit high-speed digital signals whose base clock is several hundred MHz or greater, consideration must be given to harmonic components of a GHz order.
It is known that dielectric loss and the skin effect impede the transmission of signals of a GHz order on a printed circuit board. Moreover, loss when the signals pass through plural layers through via holes for interconnecting different plural layers in a multilayer board also cannot be ignored.
According to the analyses of the present inventors, attenuation is particularly remarkable when the signals pass, through via holes, through plural layers such as a power layer and a ground layer sandwiched between two signal wiring layers. This is due to the following reasons.
Namely, in the transmission of signals in a printed circuit board, an apparent mirror current of the signal current flowing through the signal wirings flows symmetrically on reference surfaces such as the power layer and the ground layer adjacent to the signal wiring layers; in actuality, the return current distributively flows on the reference surfaces in accordance with the spread of electric power wires between the signal current and the mirror current. In a case where signal wirings wired to both sides of the printed circuit board are connected through via holes so as to penetrate plural reference surfaces, the signal current is impeded and attenuated when the return current cannot flow across these reference surfaces.
Also, when the return current cannot flow through the reference surfaces, a distant electromagnetic field is not cancelled due to the return current being incomplete. It has become apparent from the analyses of the present inventors that, in this case, common mode radiation arises.
It should be noted that signal attenuation becomes greater the smaller the connection between the two reference surfaces is. Namely, signal loss becomes greater the larger the interval between the two reference surfaces is.
Electromagnetic noise radiating from various types of electronic devices such as information devices, which has conventionally been a problem, is thought to result mainly from the signal wires of clock signals on the printed circuit board and digital signals synchronized with the clock signals. For this reason, various measures to prevent electromagnetic radiation have been adopted with respect to signal wires on the printed circuit board and wire harnesses connected to the signal wires.
For example, measures such as adding a damping resistor or filter to the signal output wires to take the edge off the rise and fall of the output signals, and disposing a guard pattern with a ground potential in the vicinity of the signal wires to make the return current loop smaller, have been widely and commonly conducted.
Also, among the electromagnetic waves observed in printed circuit boards, there are electromagnetic waves whose frequency distribution is different from that predicted from the current distribution on the signal wires and which have a sharp peak at a specific frequency without relation to the nature of the signal wires. It is known that the main cause of these electromagnetic waves lies in the electrical power system and not in the signal wires of the printed circuit board. Specifically, it lies in electrical resonance generated in the opposing power layer and ground layer.
As mentioned previously, when signals are transmitted at a high speed at a GHz order, loss when the signals pass through plural layers through via holes cannot be ignored. The affect of common mode radiation resulting from the return current being cut off is also large.
The present inventors analyzed the attenuation of signals traveling back and forth through two via holes in a case where the distance between two reference surfaces was 900 μm. As a result, it was revealed that 2.5 dB attenuation is likely to occur in signal components with a frequency of 2 GHz. In other words, it was understood that transmission of a GHz order in such a case is hardly practical.
In a printed circuit board of a structure where two ground layers are sandwiched between signal wiring layers, improvement can be made to a certain extent by disposing, in the vicinity of a signal-use via hole, a connection-use via hole for connecting plural ground layers to ensure that the return current flows through the plural ground layers. However, even in a case where a connection-use via hole is disposed in the vicinity of a signal-use via hole (e.g., a distance of 1.5 mm), the return current does not return in a GHz order and the effect of the improvement is small.